Move some drawing code from `area.c` and `ed_util.c` into `ed_draw.c`.
This is to support the new generic slider that wil be used in T81785.
No functional changes.
Reviewed By: #animation_rigging, #user_interface, Severin, sybren
Maniphest Tasks: T81785
Differential Revision: https://developer.blender.org/D9313